Kinetics studies how fast reactions happen (rate equations, mechanisms), while Equilibria examines the position of reversible reactions — both essential for understanding industrial chemistry.
Rate equation: rate = k[A]^m[B]^n. Order (m, n): determined experimentally, NOT from stoichiometry. Zero order: rate independent of [A]. First order: rate ∝ [A]. Second order: rate ∝ [A]². Half-life: constant for first order (t₁/₂ = ln2/k). Rate-determining step: slowest step in mechanism — species in this step appear in rate equation. Arrhenius equation: k = Ae^(-Ea/RT). Higher temperature → larger k → faster reaction.
Kc = [products]ⁿ / [reactants]ᵐ at equilibrium (concentrations raised to stoichiometric powers). Kp: same using partial pressures (for gaseous equilibria). Pure solids and liquids excluded from expressions. Large K → products favoured. Small K → reactants favoured. Le Chatelier\'s principle: system opposes changes to conditions. Temperature affects K value: increase T → K shifts in endothermic direction. Pressure/concentration changes: K unchanged, position shifts.
Proposed mechanism must: (1) be consistent with overall equation, and (2) be consistent with rate equation. The rate-determining step determines the rate law. Example: if rate = k[A][B] and mechanism is Step 1 (slow): A + B → C, Step 2 (fast): C + D → E, then the rate equation matches step 1. Catalysts appear in one step and are regenerated — they do NOT appear in the overall equation but speed up the rate-determining step.
The rate equation is determined experimentally, not theoretically. The stoichiometric equation shows the overall reaction, which often occurs in multiple steps (mechanism). Only the slowest step (rate-determining step) determines the rate. The order with respect to each reactant reflects how it appears in the rate-determining step, not the overall equation. For example, the reaction 2NO₂ + F₂ → 2NO₂F has rate = k[NO₂][F₂] (both order 1), matching the slow first step: NO₂ + F₂ → NO₂F + F (fast step: NO₂ + F → NO₂F). The overall stoichiometry gives 2 for NO₂, but the order is 1.
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